For example, to manufacture a structural part having a closed section (a closed structure part), such as a side member or a side door of a vehicle (e.g., a motor vehicle), a plurality of sub-parts of the closed structure part are formed from a metal plate (e.g., a steel plate) using press forming (i.e., press sub-parts). Thereafter, one of the press-formed sub-parts is attached to another press sub-part, and the two press sub-parts are fixedly joined to each other by, for example, hemming or welding. In this way, a closed structure part is manufactured from a plurality of press sub-parts.
An example of such a closed structure part is a door structure of a vehicle described in Japanese Unexamined Patent Application Publication No. 2007-176361. The door structure of a vehicle described in JP 2007-176361 includes an inner panel and an outer panel each having a concave shape. The inner panel has, in an edge portion thereof, a hemming flange bent towards the outer panel. The hemming flange is bent to sandwich the edge portion of the outer panel. In this way, the inner panel is hemming joined to the outer panel.
In addition, Japanese Unexamined Patent Application Publication No. 5-228557 describes a hemming machine for joining an outer panel to an inner panel by hemming (press hemming) (refer to Paragraphs [0002] and [0003] and FIGS. 5 through 10). To join an outer panel to an inner panel, the hemming machine places the inner panel and the outer panel so that the inner panel and the outer panel overlap each other, brings a pre-hemming steel into contact with the top end portion of the hemming flange of the outer panel, and urges the top end portion in the diagonally downward direction to bend the top portion. Thereafter, the hemming machine moves the pre-hemming steel downward to further bend the hemming flange. The edge portion of the inner panel is sandwiched by the hemming flange of the outer panel. In this way, the outer panel is joined to the inner panel by hemming (hemming joint).
In addition, to manufacture a front side member, which is a closed structure part used for absorbing a shock occurring when the vehicle collides with an object, the flange portions formed for a plurality of press parts are firmly joined with one another using welding, such as spot welding, laser welding, or arc welding.
When manufacturing the above-described closed structure part having a closed section, a plurality of press sub-parts of the closed structure part are formed from, for example, a steel plate by pressing. Thereafter, the press sub-parts are placed to overlap one another. The flange portions of the press sub-parts are joined by hemming or welding. Thus, a plurality of press sub-parts are assembled into the closed structure part.
However, in general, the weight of a closed structure part having a closed section increases as the number of press sub-parts of the closed structure part increases. That is, if the number of the press sub-parts increases, a connection flange portion is needed for each of the press sub-parts. In addition, such a flange portion needs to be formed on either end of the press sub-part with an inner space therebetween. Accordingly, as the number of the press sub-parts increases, the ratio of the weight of the flange portions to the entire weight of the closed structure part increases. As a result, the weight of the closed structure part is increased.
In addition, such a closed structure part is manufactured through at least a press step to form a plurality of press sub-parts of the closed structure part using dedicated press forming dies and a hemming step to join the press sub-parts to one another by hemming. In recent years, to reduce the manufacturing cost of closed structure parts, it has been required to manufacture closed structure parts more efficiently than ever.
Accordingly, it could be helpful to provide a method and an apparatus capable of reducing the number of sub-parts of a closed structure part and the number of steps for manufacturing the closed structure part and, therefore, efficiently manufacturing the closed structure part. It could also be helpful to provide a lightweight closed structure part by reducing the number of sub-parts.